Atrial fibrillation (AF) is the most common cardiac rhythm disorder and it affects an estimated 2.3 million adults in the United States, the majority of who are over the age of 65 years. Far from benign, AF can lead to stroke, tachycardia-induced cardiomyopathy, and congestive heart failure. AF accounts for about 15% of all is strokes that occur each year in the United States. The number of patients with AF is increasing throughout the industrialized world as the population ages. In the United States, the prevalence of AF is expected to grow 2.5-fold to 5.6 million by 2050, and over half of those afflicted will be aged 80 or older As the burden of this disorder grows, increased emphasis will be placed on developing more effective ways to treat AF to reduce its associated morbidity and mortality (James L. Cox, MD, Surgical Management of Atrial Fibrillation, Medscape Cardiology, May 2005). Implanted pacemakers and intracardiac cardioverter defibrillators (ICD) deliver therapy to patients suffering from various heart-diseases (Clinical Cardiac Pacing and Defibrillation, 2nd edition, Ellenbogen, Kay, Wilkoff, 2000). Dual chamber pacemakers pace the right ventricle with synchrony to the sensed atrial event, with a given delay, the atrioventricular (AV) delay. Cardiac Resynchronization Therapy (CRT) devices, i.e. biventricular pacemakers, pace both ventricles, and also synchronize according to the sensed atrial event signal.
However, a significant proportion of patients having a dual chamber pacemaker or biventricular pacemaker suffer also from atrial fibrillation episodes, possibly long lasting. These patients will not benefit from their dual chamber pacemaker or biventricular pacemaker during an episode since both devices synchronize according to the sensed atrial events that are not reliable during atrial fibrillation episodes, and hence will not deliver physiologic pacing during atrial fibrillation episodes.
In PCT publication WO0038782 a pacing system is disclosed, featuring a mode switching feature and ventricular rate regularization (VRR) function capable of stabilizing or regularizing ventricular heart rate during chronic or paroxysmal atrial tachyarrhythmia. The VRR function accomplishes this result by adjusting pacing rate according to the pattern of the most recent series of sensed or paced ventricular events.
In US2005187585 patent application, a method is disclosed for adaptively smoothing ventricular rate during atrial fibrillation (AF). According to this method, the pacing delivered by a pacing device is switched to a non-atrial synchronized mode when AF is detected. The ventricular escape interval (VEI) is modulated beat-by-beat around a physiological interval zone (PIZ), which is determined by the pre-arrhythmia ventricular rate or the output of rate responsive sensor.
Selective atrioventricular nodal (AVN) vagal stimulation (AVN-VS) has emerged as a novel strategy for ventricular rate (VR) control in atrial fibrillation (AF). AVN-VS is delivered to the epicardial fat pad that projects the parasympathetic nerve fibers to the AVN. Although AVN-VS preserves the physiological ventricular activation sequence, the resulting rate is slow and irregular. This issue is discussed in “Ventricular Rate Control by Selective Vagal Stimulation Is Superior to Rhythm Regularization by Atrioventricular Nodal Ablation and Pacing During Atrial Fibrillation”, by Shaowei Zhuang, et al, Circulation. 2002; 106:1853. The authors indicate that the AVN-VS although producing a superior hemodynamic performance as compared to an ablation and pacing approach, results in irregular ventricular contractions rate, i.e. irregular sensed R-R intervals.
P. Taggart and P. Sutton argue In “Termination of Arrhythmia by Hemodynamic Unloading”, published in Cardiac Mechano-Electric Feedback & Arrhythmia, by Kohl, Sachs and Franz, Elsevier Saunders, 2005, that hemodynamic unloading should be anti arrhythmic: “ventricular or atrial unloading should tend to be protective against focal tachycardia caused by triggered activity. The foregoing theoretical predictions are supported by several studies in different animal models in which arrhythmia were induced by increased stretch or volume loading.”
The various methods presented above for rate regulation during atrial fibrillation episodes are based on heart rate regulation and do not take into account the exact cardiac cycle timings when pacing the ventricles during atrial fibrillation while the AVN-VS method preserves the physiological ventricular activation sequence but produces irregular ventricular contractions.